This is a method for making turbine blades for combustion turbines, including aircraft turbines, marine turbines, and land-based gas turbines. This invention utilizes a two step solidification to produce a fine grained (non-single crystal) structure in the root section and a single crystal structure in the airfoil section.
Gas turbine engines operate by extracting energy from high temperature, high pressure gas as it expands through the turbine section. The actual rotating components which are driven by the gas are generally manufactured from nickel-based superalloys and are commonly known as blades. They consist, as shown in FIG. 1, of a contoured airfoil which is driven by the hot gas stream and of a machined root which connects to the turbine rotor. Due to the nature of the carnot cycle, gas turbines operate more efficiently at higher temperatures and there has thus become a demand for materials which are able to withstand higher temperatures. The major mechanical modes of failure for turbine blades, such as in aircraft engines and in land-based turbine generators, at high temperatures have been thermal fatigue and the lack of creep rupture resistance. Both of these problems may be reduced by elimination of grain boundaries which are transverse to the major stress axis. Thus, single crystal and directionally solidified blades are known to display significantly improved high temperature strength.
While large grain sizes improve the desired properties in the very high temperature regime, at low temperatures certain mechanical properties are improved by lower grain size. Specifically, the root section of a turbine blade runs at considerably lower temperature than the airfoil and is, essentially, subjected to fatigue loading. Consequently, the optimum structure for airfoil and root sections of the blades are very different and, in conventional airfoils, some compromise must be accepted in at least one of these sections. The optimum properties would be obtained if a hybrid blade structure were produced with a directionally solidified airfoil and a fine grained root section.
In U.S. Pat. No. 4,184,900, issued Jan. 22, 1980 to Erickson et al., two different directionally solidified sections are produced to obtain different properties in the airfoil and root sections. In U.S. Pat. No. 3,790,303, issued Feb. 5, 1974 to Endres, a eutectic alloy is used to produce a hybrid turbine blade (bucket) having an airfoil which is directionally solidified and a non-oriented structure in the root. It should be noted that Endres' eutectic composition avoids composition inhomogenuities which would result if non-eutectic compositions were used in such a method.
In U.S. Pat. No. 3,695,941 to Green discloses a method of preparing solid eutectic material by controlled solidification from the melt including the application of a steady directional magnetic field to the liquid-solid interface. During exposure of the melt to the field, no external source of electric current is electrically connected to the material. The Piercey U.S. Pat. No. 3,494,709 issued Feb. 10, 1970 discloses a single crystal metallic part for a gas turbine. The whole part, including the root, is single crystal. The Alberney U.S. Pat. No. 3,981,345 discloses a method for continuously casting metal in which the metal is subjected to a magnetic field during solidification.
This invention is a turbine blade having a hybrid grain construction and which is fabricated using alloy compositions which are non-eutectic. The airfoil sections are single crystal while the root section has a fine grained, non-directionally solidified structure.
The process utilizes solidification at a slow enough rate to allow growth of the single crystal beginning at the airfoil end, with monitoring of the solidification. When the solidification reaches the interface between the airfoil and root sections, magnetic stirring is commenced to eliminate the inhomogeneous zone adjacent to the just-solidified portion. Cooling is then increased to a rate faster than that at which either growth of a single crystal or directional solidification occurs. Grain boundary strengthener (preferably carbon) is preferably added at about the same time stirring is begun. Thus, a blade is produced with a single crystal airfoil section and a fine grained root section, and without a substantially inhomogeneous portion at the interface between the airfoil and root sections.